Heat-sensitive recording papers comprise a support, e.g., paper, synthetic paper, plastic films, etc., having provided thereon a heat-sensitive color forming layer capable of developing a color upon heating, and have been widely used in various recording systems, such as electric calculators, thermal printers of terminals of computers, thermal pens of medical instruments, recorders, heat-sensitive facsimiles, automatic ticket vending machines, and the like. The heat-sensitive recording materials have generally been produced by separately pulverizing and dispersing a color forming agent, such as colorless or light-colored leuco dyes, lactone, lactam or spiropyran color forming dyes, and the like, and a developer capable of causing the color forming agent to develop a color upon heating, such as naphthol, catechol, resorcinol, 4,4'-isopropylidenediphenol, 4,4'-cyclohexylidenediphenol, benzoic acid, p-phenylphenol, and the like, in a ball mill, an attritor, a sand mill, etc., adding a resin binder, and, if necessary, a surface active agent, a defoaming agent, a wax, an inorganic pigment, etc., and coating the resulting composition to a support, such as a sheet of paper, followed by drying.
The thus produced heat-sensitive recording paper is heated with a thermal head or a thermal pen made of a resistance heating element, nichrome wire, etc., to form a color image.
In order to obtain high color densities with smaller energy or to reduce adhesion of the scum to the thermal head, etc., it has been proposed to provide an intermediate layer comprising an organic or inorganic pigment between the paper support and the heat-sensitive recording layer as described in Japanese Pat. Application (OPI) Nos. 23545/79 and 86792/81 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
Also known is a method of forming an intermediate layer comprising an inorganic pigment having an oil absorption of not less than 60 ml/100 g, e.g., a calcium carbonate, as proposed in Japanese Patent Publication No. 52915/82 and a method of forming an intermediate layer comprising a urea-formaldehyde resin as an organic pigment as described in Japanese Patent Application (OPI) No. 103892/82.
Further, in order to improve image quality, i.e., dot reproducibility of a heat-sensitive head, a method in which a heat-sensitive layer is provided on a lightweight coated paper having an optical surface roughness (Rp) of not more than 8 .mu.m has been proposed as disclosed in Japanese Patent Application (OPI) No. 136492/83.
These conventional heat-sensitive recording papers, however, involve a disadvantage in that the color forming components are absorbed in the intermediate layer when recording is effected by high energy, or during preservation after the color development, thus resulting in reduction of color density. Therefore, a heat-sensitive recording material that can fully satisfy the need for both excellent color density and image quality has not yet been obtained.